I can’t really be sure if I’m actually reading both the GUI as well as the documentation wrong, but isn’t there an option to specify the internal port while doing port forwarding on a Peplink 310?
I’ll specify both scenarios here.

Hi Tim and thanks for your reply, but I don’t think you understand what I mean, or it’s me beeing utterly blind.
Your KB article doesn’t ever state the translation of ports, it’s just 80 -> 80, since you can only specify this on a single place, usually when you’re configuring Port Forwarding you get to choose a WAN port like 8080, which later gets NATed to whatever server at say port 80 instead.

And as you state, this is indeed one of the most trivial functions in these sort of boxes, I can’t really imagine Peplink beeing any less good then a D-Link for ~30EUR.